Contraction

Cooling Time is the phase of the molding cycle in which the already-packed part loses heat until it is rigid enough to be ejected without distortion. It typically accounts for 50 – 70 % of the total cycle time, so it is the first optimization target. ## Approximate calculation The classic Ballman & Shusman formula scales quadratically with wall thickness: > t_cool ≈ (s² / α·π²) · ln[(4/π) · (T_melt − T_mold)/(T_eject − T_mold)] Where s = wall thickness (m), α = thermal diffusivity (m²/s), T_melt / T_mold / T_eject = temperatures (°C). In practice: doubling the wall quadruples cooling time. ## Typical values - 1 mm wall: 2 – 5 s - 2 mm wall: 8 – 15 s - 3 mm wall: 18 – 30 s - 4 mm wall: 30 – 50 s ## Factors that affect cooling - Mold temperature (colder → faster, until condensation limit) - Resin thermal diffusivity (PE and PP slower than ABS or PS) - Cooling-channel design (proximity, balance, flow rate) - Coolant (water + glycol, conformal cooling) - Wall thickness (dominant factor) ## Optimization Conformal cooling channels following 3D part geometry (built by DMLS), reduce thickness in CAD, separate mold temperature controllers per circuit, and mold temperature monitoring with embedded thermocouples.

Dimensional Stability is a molded part's ability to keep its critical dimensions within tolerance over time and under service conditions (temperature, humidity, load). It is a combined property of resin, design and process. ## Influencing factors - Resin type: amorphous (PC, ABS, PMMA) are the most stable; semi-crystalline (PP, PA, POM) show post-shrinkage - Hygroscopicity: PA absorbs 1 – 8 % moisture, dimensions can change up to 2 % - Reinforcement: glass fiber cuts directional shrinkage 50 – 70 % but causes warpage - Residual stress from process (poor hold, asymmetric cooling) - Tg and service T: above Tg the polymer relaxes residual stress ## Most stable resins (ranked) 1. Glass-fiber-reinforced PC 2. PEI / PSU 3. PC unfilled 4. ABS 5. POM (stable but post-shrinkage) 6. PA (poor unless dry) 7. PP / PE (least stable, high thermal coefficient) ## Tests and verification - ISO 75 HDT (Heat Deflection Temperature) - ASTM D696 coefficient of thermal expansion - ISO 62 dimensional stability under humidity - Longitudinal measurement at 24 h, 7 days, 30 days post-mold ## How to improve Symmetric cooling, hold until gate seal, annealing on technical parts, avoid uncontrolled regrind, and add glass fiber or mineral fillers in tight-tolerance parts.

A plastic is a synthetic or semi-synthetic material whose backbone is a polymer — long chains of repeating molecular units — usually combined with additives that tune color, stability, flow and strength. The word "plastic" refers to its plasticity: when heated it can be shaped and then set into a solid part, which is exactly what injection molding exploits. ## Plastic vs polymer vs resin - polymer: the pure long-chain molecule (e.g. polyethylene). - Plastic: the usable material = polymer + additives, the everyday term for the finished compound. - resin: in a molding shop, the pelletized plastic fed into the machine. In practice molders use "resin," "plastic" and "material" almost interchangeably. ## The two families that matter for molding - thermoplastic: softens when heated and re-solidifies when cooled, reversibly — so it can be melted, molded, reground and re-melted. Virtually all injection molding uses thermoplastics. - thermoset: cures into a permanent crosslinked network and cannot be re-melted (e.g. epoxy, phenolic). Molded by different processes. ## Structure that drives behavior Thermoplastics are either amorphous (random chains — PC, ABS, PS: gradual softening, lower shrink, often clear) or semi-crystalline (ordered regions — PP, PA, POM: sharp melting, higher shrink, chemical resistance). This structure sets melt behavior, viscosity, shrinkage and where the molding process window lives. ## Related terms - See also: polymer, thermoplastic, thermoset, resin, additive ## What is plastic made of? A base polymer — long repeating molecular chains, usually from petrochemicals or increasingly bio/recycled sources — blended with additives such as stabilizers, colorants, lubricants and reinforcements to reach usable properties. ## What is the difference between a plastic and a polymer? A polymer is the pure long-chain molecule; a plastic is the usable material made from that polymer plus additives. In molding, the pelletized plastic fed to the machine is usually called resin. ## What kinds of plastic are used in injection molding? Almost always thermoplastics, which melt and re-solidify reversibly — split into amorphous grades (PC, ABS, PS) and semi-crystalline grades (PP, PA, POM); thermosets cure permanently and use other processes.

Mold (Tool) is the mechanical assembly of plates, cavities, injection system and cooling that shapes the molded part. It is the single most expensive asset of the operation (10,000 – 500,000 USD) and its design dictates everything: cycle, quality, productivity and unit cost. ## Main components - Cavity plate (fixed half): nozzle side, usually houses the cavity - Core plate (moving half): ejector side, holds the core and ejector pins - Injection system: sprue, runners, gates (cold or hot) - Cooling system: water/glycol channels, conformal in premium molds - Ejection system: pins, sleeves, stripper plates, slides for undercuts - Standard components: leader pins, bushings, retainers, sensors - Replaceable inserts in wear areas ## Mold types - Single-cavity: prototypes, large parts, low production - Multi-cavity (2/4/8/16/32+): mass production - Family mold: different cavities for parts of the same assembly - Cold runner: with cold runners separated each cycle - Hot runner: no runner scrap, shorter cycles - Stack mold: two levels of cavities to double capacity - Two-shot / multi-material: two resins in the same part ## Mold materials - P20 (pre-hardened steel): standard for medium production, easy to machine - H13: hardened inserts, high thermal-wear resistance - S136 (stainless): polished cavities, corrosion resistance (PVC, PET) - Aluminum (7075): prototype or low-volume molds - NAK80: mirror polish without distortion ## Typical service life - Aluminum: 5,000 – 50,000 cycles - P20: 100,000 – 1,000,000 cycles - Hardened H13: 1 – 10 million cycles - Carbide / TZM in hot-runner gates: up to 50 million ## Critical maintenance Cleaning after every production run, vent inspection, lubrication of pins and guides, monitoring of cooling channels (scaling), and repair of cavity damage before it spreads.